Extruder

ABSTRACT

In an extruder for continuous working and/or processing of flowable substances having at least two parallel, corotating shafts equipped with intermeshing screw elements whose face is limited by circular arcs corresponding to the outside screw diameter, the screw core diameter and at most the center distance of the screw elements, and guided on circular segments of the extruder housing that are parallel to the shafts, at least one screw element ( 1 ) has a plurality of concentric ring portions ( 8 ) spaced a distance apart and formed by grooves ( 12 ) turned into the screw element ( 1 ).

This invention relates to an extruder for continuous working and/or processing of flowable substances according to the preamble of claim 1.

For continuous working, i.e. in particular mixing and kneading, of flowable, i.e. liquid, viscous, plastic or particulate, substances, so-called kneading blocks consisting of disk cams disposed offset in the manner of a spiral staircase are used in twin-shaft extruders (cf. DE-C-813154). The substance is pressed through a narrow tapered gap between the disk cams and the extruder housing, which can result in different heating and partial overstressing by shearing of the substance, and further causes locally above-average wear.

DE-A-2 003 593 already discloses a twin-shaft extruder according to the preamble of claim 1 wherein the screw flight of one screw is sealed by a sealing bar while the other screw has a recess at the place in the screw crest that moves past said sealing bar upon rotation. Here, too, the material blocked with the sealing bar in the flight of the screw is subjected to high stress, while the screw with the recess makes only a small contribution to working the substance.

The problem of the invention is to convert the driving energy of the extruder optimally into uniform working and treatment of the substance.

This is obtained according to the invention with the extruder characterized in claim 1. The subclaims render advantageous embodiments of the invention.

According to the invention, the extruder has at least one screw element provided with a plurality of ring portions disposed at a distance apart concentrically or coaxially with the screw shaft and formed by grooves turned into the screw element.

The screw portions of the screw element between the ring portions first cause a pressure buildup which then urges the substance through the annular gap between extruder housing and the ring portions with shearing action and elongation, and the pressure is reduced again. The recurring sequence of shear gap passage, pressure buildup, shear gap passage, etc., on the screw elements causes a defined stress on the substance and thus a uniform stress, without unduly stressing the substance or the screw.

The annular and/or shear gap between the ring portions and the concave circular segments of the extruder housing can have a different height to produce a sufficient mixing and/or kneading effect in accordance with the particular substance. The ring portion might for this purpose correspond only to the core diameter of the screw shaft. However, the annular gap preferably has a height of from 10 percent to 90 percent of the flight depth of the screw. Especially preferred is a diameter of the ring portions that corresponds approximately to the center distance of two adjacent shafts.

Before the substance is stressed during passage through the annular or shear gap, it must be transported a certain conveying distance by a screw portion to build up the required pressure. For this purpose the screw portions located between two adjacent ring portions generally have a length of at least {fraction (1/10)}, preferably at least ⅕, of the screw diameter. The ring portions are preferably formed by grooves turned into the screw element. The turned grooves preferably have a depth of for example ½ or less of the flight depth. The angle of the flanks of the turned grooves can be for example 30 to 90 degrees. Preferably, oblique grooves are turned, in particular with an angle of about 60 degrees to the shaft axis.

By stock removal on the screw crest and flanks, the screw element can be provided with further portions. Thus, in particular a mixing section with substantially neutral conveying action can be provided by stock removal.

Thus, the invention provides in particular for multi-shaft extruders a screw element which can meet the most diverse requirements of process engineering from defined longitudinal mixing of large volumes in the deciliter range to intensive and lasting action in the centi- to milliliter range.

After the annular gaps the screw flight can continue at the same pitch angle. That is, the screw portions of the screw element can form a continuous screw flight, apart from the turned interruptions in the area of the ring portions.

The ring portions permit additional dispersing surfaces to be gained. A substantial enlargement of dispersing surface can moreover be obtained if the screw portions between the ring portions are disposed at a progressive angular offset from each other with the same sense of rotation, for example at an angular offset by half the flight angle. The angularly offset screw portions form faces angularly offset in step-like fashion as additional dispersing surfaces.

The inventive extruder can have only two screw shafts. However, it can also have at least three shafts disposed along a circle or circular arc with the same central-angle distance within a cavity of the extruder housing, the extruder housing being provided on the radially inner and outer sides of the cavity with concave circular segments parallel to the extruder axis, on which the screw elements are tightly guided, as described for example in EP-B-0788867. The inventive screw elements thus permit substance to pass through the ring of shafts from one side of the cavity to the other.

In the following, the inventive extruder will be explained in more detail with reference to the drawing, in which:

FIG. 1 shows a cross section through a twin-shaft extruder with screw elements closely intermeshing on the whole circumference;

FIGS. 2 a to 2 c show a side view, perspective view and view of the transverse profile of a first embodiment of the screw element;

FIGS. 3 a to 3 b show a side view, perspective view and view of the transverse profile of a second embodiment of the screw element; and

FIG. 4 shows a perspective view of a third embodiment of the screw element; and

FIG. 5 shows a perspective view of a fourth embodiment of the screw element

According to FIG. 1, screw elements 1 have transverse profile 2 composed of three circular arcs A-B, E-F and A-E. Circular arc A-B has a diameter corresponding to outside screw diameter D, circular arc E-F a diameter corresponding to screw core diameter d, and circular arc A-E a diameter whose radius corresponds to center distance Ax of the two screw elements 1 (cf. EP-B-0002 131).

Intermeshing screw elements 1 of the twin-shaft extruder are fitted so as to rotate in unison on two parallel, corotating shafts 3 guided on circular segments 4 of extruder housing 6 that are parallel to screw shafts 3, so that two wedges 5 are formed.

According to FIGS. 2 a to 2 c, double-flighted screw element 1 has three ring portions 8 concentric with shaft axis 7 and disposed a distance apart.

Ring portions 8 are provided on the side of screw element 1 facing the outlet of the extruder here, with respect to the conveying direction shown by arrow 9, i.e. in conveying direction 9 there is long screw portion 11 a, followed after first two ring portions 8 by two shorter screw portions 11 b and 11 c, and after third ring portion 8 by even shorter screw portion 11 d.

While screw portion 11 a corresponds approximately to screw diameter D, the distance between ring portions 8 corresponds to about one third of the screw diameter, and the distance of the face of screw segment 11 d from adjacent ring portion 8 to only about ⅙ D.

Ring portions 8 are obtained by grooves 12 turned into screw element 1. Angle α of flanks 13 of grooves 12 to shaft axis 10 is about 60 degrees.

The height of annular gaps 14 between ring portions 8 and extruder housing 6 is according to FIG. 2 c about half of flight depth t, i.e. half the difference between core diameter d and outside screw diameter D (FIG. 1). Diameter dr of ring portions 8 thus corresponds approximately to center distance Ax of shafts 3 from each other.

Screw element 1 according to FIGS. 3 a to 3 c differs from that according to FIGS. 2 a to 2 c substantially in that, instead of screw portion 11 a, portion 15 with neutral conveying action is provided which is formed by corresponding stock removal on the screw crest of screw element 1.

Screw element 1 according to FIGS. 3 a to 3 c is suitable in particular for an extruder having three or more screw elements disposed along a circle or circular arc with the same central-angle distance within a cavity of an extruder housing and dividing the cavity into an inside space and outside space. As mentioned, material must be balanced between the inside and outside spaces in such ring extruders. While ring portions 8 and short screw portions 11 b, c and d of the screw element according to Figures 3 a to 3 c inhibit the product flow, portion 15 permits passage from one process space to the other.

The screw element according to FIG. 4 differs from that according to FIGS. 3 a to 3 c substantially in that only two ring portions 8 are provided, one ring portion 8 being disposed between one face of screw element 1 and portion 15 with neutral conveying action, and two screw portions 11 e and f separated by second ring portion 8, and further screw portion 11 g being provided between the face of screw element 1 and ring portion 8 on portion 15.

In screw element 1 according to FIGS. 2 a to 2 c, FIGS. 3 a to 3 c and FIG. 4 a continuous screw flight is formed, as indicated by dashed line 16 in FIG. 2 a, which is interrupted only by turned grooves 12 with ring portions 8.

In contrast, in the embodiment according to FIG. 5 screw portions 11 h, i, j . . . between ring portions 8 are disposed at a progressive angular offset from each other with the same sense of rotation. 

1. An extruder for continuous working and/or processing of flowable substances having at least two parallel, corotating shafts equipped with intermeshing screw elements whose face is limited by circular arcs corresponding to the outside screw diameter, the screw core diameter and at most the center distance of the screw elements, and guided on circular segments of the extruder housing that are parallel to the shafts, characterized in that at least one screw element (1) has a plurality of concentric ring portions (8) spaced a distance apart and formed by grooves (12) turned into the screw element (1).
 2. An extruder according to claim 1, characterized in that the axial distance of the ring portions (8) of the screw element (1) is not greater than the outside screw diameter (D).
 3. An extruder according to claim 1, characterized in that the axial distance of the ring portions (8) of the screw element (1) is at least {fraction (1/10)} of the screw diameter (D).
 4. An extruder according to claim 1, characterized in that the height (h) of the annular gap (14) between the ring portions (8) and the circular segments (4) of the extruder housing (6) corresponds to one quarter to three quarters of the flight depth (t).
 5. An extruder according to claim 1, characterized in that the diameter (d) of the ring portions (8) corresponds approximately to the center distance (Ax) of the shafts (3).
 6. An extruder according to claim 1, characterized in that the flanks (13) of the ring portions (8) extend obliquely to the shaft axis (7).
 7. An extruder according to claim 1, characterized in that the screw portions (11 h, i, j . . . ) between the ring portions (8) are disposed at a progressive angular offset from each other with the same sense of rotation.
 8. An extruder according to claim 1, characterized in that the screw portions (11 a, b, c . . . ) of the screw element (1) form a continuous screw flight(16), apart from the interruption in the area of the ring portions (8).
 9. A device according to claim 1, characterized in that the screw element (1) additionally has at least one portion (15) with neutral conveying action. 